1. Field of the Invention
The present invention relates a radiographic, digital image processing system for processing a radiographic, digital image.
2. Related Background Art
In recent years, development has been made in radiographic image taking systems arranged in such structure that radiations such as X-rays are radiated onto a subject, a radiographic image as a transmitted image of the subject is picked up directly by a solid state image sensing device, and an image signal corresponding to the radiographic image thus picked up is displayed as a visible image on a CRT (Cathode Ray Tube) display device or the like; or, the image signal corresponding to the radiographic image thus picked up is digitized, image processing is carried out in the digital data state, and it is printed out.
In the radiographic image taking systems described above, photographing portions differ depending upon their photographic purposes, and in the image processing step for visualizing the radiographic image the optimum density and gradation vary every image of processed portion. Therefore, it is necessary to carry out different image processing operations among images of the respective portions.
Meanwhile, computer networks are also spreading recently in medical treatment facilities. Under such circumstances, in addition to the radiographic image taking systems described above, there are also cases where a single image processing device is used to process pieces of radiographic image information taken by different radiographic image taking devices such as a radiographic image taking device using a photo-stimulable phosphor sheet (CR) and a radiographic image taking device using an image intensifier (DR) or the like and the radiographic image information thus processed is outputted to either one of different output devices such as the CRT display device, a film imager device, and a dry printer device.
In the above-stated cases, the operator himself manipulating the image processing device had to carry out a setting operation including a plurality of procedures in order to match the image processing carried out in the image processing device with each taking device or with each output device. This operation was very troublesome to the operator.
When the chest part is photographed, an area of interest varies depending upon circumstances; for example, the area of interest is the pulmonary field in some cases or is a bone part in other cases. Depending upon whether the actual area of interest is the pulmonary field or the bone part, the operator himself had to manipulate an input device such as a mouse or a touch panel so as to carry out the image processing operation in the density and gradation, different between the areas, and to set the image processing device so as to carry out the image processing operation adapted for the area of interest in the radiographic image. These operations also took some time of the operator.
When the subject was a patient provided with a radiation-absorbing auxiliary device such as a pacemaker or a fitting for fixing a bone or the like in the body, the signal level of the part including the auxiliary device or the fitting became lower than that of the part around it. It was, therefore, difficult in some cases to properly carry out the above-stated image processing such as gradation processing.
For example, a radiographic chest image as a photograph of the chest was composed of image areas the pulmonary field readily transmitting radiations and showing high density values and image areas of mediastinal parts hardly transmitting radiations and showing low density values, so that the dynamic range was very wide of the density values of pixels constituting the radiographic image. It was thus considered to be difficult to obtain an image allowing both the pulmonary field and the mediastinal parts to be observed simultaneously in good order on the same radiographic chest image.
A conventional method for solving the above problem was a process for compensating the radiographic image by use of a filter called "self-compensating digital filter" (Mitsuhiro Anan et al., JAPANESE JOURNAL OF RADIOLOGICAL TECHNOLOGY, Vol 45, No. 8 (Aug 1989), p1030) so as to improve the image area desired to be observed by a doctor (the area of interest).
The self-compensating digital filter described itu above is a filter defined by Eq. (1) and Eq. (2) below: EQU S.sub.D =S.sub.org +f(S.sub.US) (1)
##EQU1##
where S.sub.D is a pixel value after the compensation (after the processing), S.sub.org is an original (input) image value, S.sub.US is an average pixel value obtained in such a way that a mask having the size of M pixels.times.M pixels is moved on an original image (input image) and an average of pixel values existing in the mask is calculated at each moving portion, and f(x) is a function to represent the function curve as illustrated in FIG. 1.
Described below are characteristics of the function f(S.sub.US) as illustrated in FIG. 1. Let "BASE" in the figure be a density reference value and "SLOPE" be a compression factor. First, the function f(S.sub.US) is "0" in the density region of pixel values of "S.sub.US &gt;BASE", and the function f(S.sub.US) monotonically decreases at the rate of the compression factor "SLOPE" down to the end point of the density reference value "BASE" in the density region of pixel values of "0.ltoreq.S.sub.US.ltoreq.BASE". The following effect is thus achieved by processing the pixel values S.sub.org of the original image by the "self-compensating digital filter" shown in Eq. (1) above; "in an area with a low average density value (average pixel value S.sub.US) of image, the density values are increased to compress the dynamic range of the low density area but the contrast of fine structure is maintained in each area, so that the low density area is converted as a whole to a higher-density image with the contrast of the fine structure thereof being maintained".
In the method for compensating the radiographic image using the "self-compensating digital filter" as described above, however, predetermined values that were empirically obtained have been used as the density reference value "BASE" and the compression factor "SLOPE"; therefore, the effect of compression of the dynamic range varied, depending upon the difference in a photographing portion, the physical constitution of the patient being the subject, or a radiation dose of radiations. It was thus very difficult and troublesome to effect the optimum dynamic range compression for every photographing part, for every physical constitution of a patient being the subject, or for every radiation dose of radiations.